Hypoxia - a deficiency of oxygen reaching the tissues of the body - occurs during a variety of physiological and pathophysiological processes, including placenta development, hematopoiesis, angiogenesis, cerebral and myocardial ischemia, and tumorigenesis. In particular, solid tumors harbor hypoxic regions that are not only critical for tumor development and progression, but also associated with resistance to chemotherapy and radiation therapy. Hypoxic stress also affects cell metabolism, proliferation, and apoptosis. Given the essential role of hypoxia for cell proliferation and survival, it is crucial to understand the underlying mechanisms of hypoxic response at cellular and molecular levels. The hypoxia-inducible factor 1 alpha (HIF-1alpha) is a master regulator of oxygen homeostasis responsible for transcriptional upregulation of a variety of hypoxia-responsive genes, including vascular endothelial growth factor, nitric oxide synthases, glucose transporters, and glycolytic enzymes. Our research focuses on the mechanisms underlying HIF-1alpha activation, the role of HIF-1alpha in tumorigenesis, and the molecular basis for therapeutic targeting. Previously, we and others demonstrated that HIF-1alpha activation is regulated primarily by posttranslational modifications, resulting in increased protein stability and transcriptional activity. The HIF-1alpha level is controlled by oxygen-dependent proteolysis via the ubiquitin-proteasome pathway that targets the oxygen-dependent degradation domain (ODD) of HIF-1alpha. The HIF-1alpha E3 ubiquitin ligase contains the tumor suppressor von Hippel-Lindau (VHL) protein, which binds ODD and catalyzes polyubiquitination of HIF-1alpha. The VHL protein interacts with HIF-1alpha via a specific recognition of hydroxylated Pro402 or Pro564, both of which are modified in normoxia by a family of prolyl-4-hydroxylases functioning as oxygen sensors. To gain insights into the molecular determinant of prolyl hydroxylation, we recently identified a leucine residue (Leu-574) downstream of Pro-564 to be crucial for the VHL-mediated degradation of HIF-1alpha. Moreover, we have demonstrated that the leucine residue is required for recruiting a key HIF prolyl-4-hydrxylase. Thus, the identification of Leu-574 may provide a molecular basis for drug targeting of HIF-1alpha activity.Solid tumors contain hypoxic regions that may induce mutations of critical genes, ultimately resulting in genomic instability. The role of HIF-1alpha in tumorigenesis has been implicated by the heightened expression of HIF-1alpha and HIF-2alpha, a close member of the family, and by the correlation of their overexpression with poor diagnosis. By taking advantage of our unique understanding of the fundamental mechanisms underlying HIF-1alpha stability and transcriptional activity, we have begun to explore the role of hypoxia in tumorigenesis, drug resistance, and tumor progression. Recently, we unraveled the mechanism by which hypoxia induces cell-cycle arrest. Moreover, we proposed a novel HIF-1alpha-Myc pathway that accounts for how hypoxia regulates genes involved in drug resistance and genomic instability. We are testing this new pathway by focusing on the role of HIF-1alpha in DNA mismatch repair. Lastly, our mechanistic studies of HIF-1alpha activation may provide a molecular basis for the development of therapeutic agents that could alter HIF-1alpha activity and its downstream target gene expression during tumor development and progression.